1. Field of the Invention
The present invention relates to a vehicle headlamp assembly that employs a discharge lamp, which is especially suitable for an automobile.
2. Related Art
Nowadays a discharge lamp is often employed as a vehicle headlamp. The discharge lamp is an arc discharge lamp which generates light by means of an internal electrical discharge between a pair of electrodes in a discharge bulb (arc tube) filled with xenon. The discharge lamp can generate white light close to sunlight. Further the discharge lamp can generate light in double intensity using 70% reduced energy in comparison with a conventional halogen lamp. Accordingly the visibility is improved if the discharge lamp is employed as a headlamp of a vehicle.
In this case, the vehicle should include a lighting control circuit dedicated to the discharge lamp. The lighting control circuit includes a converter, an inverter and an igniter. The converter converts a d.c. voltage supplied by the battery into a higher voltage. The inverter converts the d.c. voltage into an a.c. voltage. The igniter generates a high voltage of few score kilovolts required for starting the discharge lamp.
When the discharge lamp is turned on, the lighting control circuit applies a high voltage of several kilovolts or few score kilovolts to the discharge lamp so that electrical discharge immediately begins. Thus the discharge lamp is instantaneously started. Thereafter the lighting control circuit keeps the discharge lamp on by supplying power of approximately 35 W to the discharge lamp.
The lighting control circuit is usually installed outside the housing of a headlamp assembly in the vehicle. In this case, a wire for transferring the high voltage should be connected between the lighting control circuit and the discharge lamp, and further a connector is required to connect the wire to the discharge lamp. Therefore the headlamp assembly cannot be miniaturized and occupies a relatively large space in the vehicle. Further a relatively large amount of manpower is required for mounting and wiring the headlamp assembly in or to the vehicle. Further a shield is required for blocking electrical noises generated in the wire, and thereby costs are increased.
Then it is proposed that the lighting control circuit is disposed in the headlamp housing so that the lighting control circuit may be directly connected to the discharge bulb. For example, the lighting control circuit is fixed to a holder (i.e., fixture) mounted to the backside of a reflector. However a lampshade, which is provided in the vicinity of the discharge bulb for partially shielding light radiated from the discharge bulb, is also mounted to the reflector. Then it is proposed that the holder is laid on the lampshade on the reflector so that the lampshade and the holder may be together fastened to the reflector. According to this construction, the headlamp assembly can be miniaturized and the wire for transferring the high voltage is not required. As a result, noises and/or the power loss due to transmission of the high voltage through the wire are prevented.
However, while the discharge bulb is lighted, the discharge bulb has a high temperature. According to the above construction, the holder is in touch with the lampshade, that is, it is thermally connected to the lampshade, and therefore lampshade-to-holder thermal resistance is low. Accordingly the heat radiated from the discharge bulb is transferred to the lighting control circuit via the lampshade and the holder.
As a result, the lighting control circuit will rise in temperature while the discharge bulb is on, and the heat load on the circuit elements of the lighting control circuit will be increased. In order to overcome this problem, circuit elements and a substrate which have high heat-resistance may be employed. However, in this case, the costs are increased and the headlamp assembly is increased in size.